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A C*-algebraic approach to quantum coding theory [Elektronische Ressource] / von Lisa Steiner


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A C*-Algebraic Approach toQuantum Coding TheoryVom Fachbereich Mathematikder Technischen Universität Darmstadtzur Erlangung des Grades einesDoktors der Naturwissenschaften(Dr. rer. nat.)genehmigteDissertationvonDipl.-Math. Lisa Steineraus FilderstadtReferent: Prof. Dr. B. KümmererKorreferent: Dr. H. MaassenTag der Einreichung: 12. Juli 2007Tag der mündlichen Prüfung: 3. September 2007Darmstadt 2008D 17iiiAcknowledgmentsI would like to express my thanks to Burkhard Kümmerer for supervising my thesis and forgiving me the opportunity to work in his research group. His way of doing mathematics hasinfluenced me deeply.Let me also thank Claus Köstler and in particular Jürgen Hellmich for many stimulating con-versations as well as for their support at all stages of my work.Many thanks to all my collegues in our group, especially Nils Gebhardt, Florian Haag, HeikeMüller, Kay Schwieger, Nadiem Sissouno, Walter Reusswig and Julia Wiskandt. I alwaysenjoyed the friendly and supportive working atmosphere.My special thanks go to my parents and sisters, as well as to Fatma, without whom I mightnot have finished this work. Last but not least, I am very grateful to my husband Feriz, whobravely manages to live with a mathematician.vContentsAcknowledgments iiiIntroduction viiPlacement viiMain Results viiiSurvey of Chapters viiiZusammenfassung xiEinordnung der Arbeit xiHauptergebnisse xiiKapitelübersicht xiiiChapter 1. Classical Coding Theory 11.



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Published 01 January 2008
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A C*-Algebraic Approach to Quantum Coding Theory
Vom Fachbereich Mathematik der Technischen Universität Darmstadt zur Erlangung des Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) genehmigte
von Dipl.-Math. Lisa Steiner aus Filderstadt
Referent: Korreferent: Tag der Einreichung: Tag der mündlichen Prüfung:
Prof. Dr. B. Kümmerer Dr. H. Maassen 12. Juli 2007 3. September 2007
Darmstadt 2008 D 17
I would like to express my thanks to Burkhard Kümmerer for supervising my thesis and for giving me the opportunity to work in his research group. His way of doing mathematics has influenced me deeply. Let me also thank Claus Köstler and in particular Jürgen Hellmich for many stimulating con-versations as well as for their support at all stages of my work. Many thanks to all my collegues in our group, especially Nils Gebhardt, Florian Haag, Heike Müller, Kay Schwieger, Nadiem Sissouno, Walter Reusswig and Julia Wiskandt. I always enjoyed the friendly and supportive working atmosphere. My special thanks go to my parents and sisters, as well as to Fatma, without whom I might not have finished this work. Last but not least, I am very grateful to my husband Feriz, who bravely manages to live with a mathematician.
Introduction Placement Main Results Survey of Chapters
Zusammenfassung Einordnung der Arbeit Hauptergebnisse Kapitelübersicht
Chapter 1. Classical Coding Theory 1. Basic Definitions 2. Higher Block Shifts 3. Higher Power Shifts 4. Sliding Block Coders 5. Linear Coders 6. Convolutional Coders
Chapter 2. Notation and Basics of Operator Algebras 1. Notation 2. Groups and Algebras 3. Inductive Limits and AF-Algebras 4. States
Chapter 3. Stabilizer Embeddings 1. The Pauli Group 2. The Stabilizer Group 3. The Stabilizer Algebra 4. Characterization ofAS 5. Stabilizer Embeddings 6. Example of a Stabilizer Embedding 7.m-Blocks of Stabilizer Embeddings 8. Example of anm-Block of a Stabilizer Embedding 9. Stabilizer Embeddings à la Ollivier and Tillich 10. Example of an Ollivier-Tillich-Embedding
vii vii viii viii
xi xi xii xiii
1 1 3 4 4 5 8
13 13 13 15 17
19 19 26 27 29 31 33 34 37 37 40
Chapter 4. A Quantum Coding Theory 1. Literature 2. Some Elements of Quantum Probability 3. The Usual Scheme of a Quantum Algorithm 4. Quantum Alphabets 5. Quantum Shifts 6. Quantum Coders 7. The New Scheme of a Quantum Algorithm
Chapter 5. Examples of Quantum Coders 1. Infinite Blocks of Stabilizer Embeddings 2. Infinite Ollivier-Tillich-Embeddings 3. Discussion
Chapter 6. On the Realization of Quantum Algorithms 1. Measurements 2. Realization of Quantum Algorithms 3. Properties of Realizations of Quantum Algorithms 4. Measurement Operator of the Grover Algorithm 5. Discussion
Appendix. Akademischer Werdegang
43 43 43 45 46 47 48 52
55 55 60 62
65 65 67 68 69 72
This work is embedded in the mathematical field of quantum information. Quantum informa-tion is one of the key technologies of the 21st century and is based on quantum mechanics. The theory of quantum mechanics was developed approximately 100 years ago. But it wasn’t until 1982 when Feynman [Fey82it might be useful to use quantum mechanics] first stated that to construct computers powerful enough to simulate quantum systems eciently. In 1985, Deutsch [Deu85] gave the first ideas for a quantum Turing machine, resulting from a quantum mechanical description of calculation processes. This is regarded as the first model of a quantum computer. His work is seen as the foundation of a more concrete definition of Bernstein and Vazirani [BV92], what lead to a subfield of quantum information theory named quantum complexity. Schumacher [Sch95] eventually defined a qubit, the quantum version of a classical bit in information theory. He defined them as pure quantum states of two-level systems, described by unit vectors of the system Hilbert space. Soon there were first ideas for physical realizations of qubits as described by Berthiaume [HS97 The] and first results in quantum computation. first major steps were Shors [Sho97] algorithms for factorization and discrete logarithm. Both are based on the problem of order finding and solving these problems eciently on a quantum computer. The next step was the quantum search algorithm by Grover ([Gro96] and [Gro97]). Quantum coding theory currently consists of two main subfields, quantum cryptography and quantum codes. In 1997, Gottesman [Got97] introduced error correcting codes. codes These work with finite tensor products of pure states, but allow a good error correcting procedure after transmission. Error correction becomes necessary as quantum states evolve with time and this evolution is usually disturbed. However, an important question of quantum information has remained unanswered. It is to find a quantum analogue of classical coding theory. This is the objective of this thesis, together with the question, to what extent existing quantum codes and algorithms make use of quantum mechanics. We try to answer these questions by using a systematical view of quantum prob-ability theory that was introduced by Kümmerer [Küm85b] and studied by both Kümmerer and Maassen, for example in [KM98] or [Maa06 follow this way of algebraization and]. We develop analogously a quantum coding theory. A recent approach of Gohm, Kümmerer and Lang [GKL06] also leads to codes in an operator algebraic setting, but is dierent to ours.
Main Results
This work has reached several results. The first is, that it was possible to find an algebraic frame in which we can formulate stabilizer codes as developed by Gottesman. This formu-lation is independent from bases of the related Hilbert spaces which usually hide what is happening. After this result, we were able to show that the independence of generators of the stabilizer group corresponds to trace-independence. This is a notion of independence that generalizes the classical notion of independence of random variables into a quantum mechan-ical setting. This leads to a new characterization of stabilizer embeddings. It also leads to the insight, that the choice of generators of a stabilizer algebra corresponds to choosing a representation of finitely many Rademacher functions in a matrix algebra. The second part of this work was to develop a quantum coding theory based on a systematical way of algebraization of classical concepts that was described in section . Our result diers in some points from what has been developed so far, mainly because we are working not only with pure but also arbitrary states. This is a natural change as there is no reason why a general quantum coding theory should be restricted to pure states and thus exclude an essential part of quantum mechanics. This approach is supported by [AKN98] and [Maa06] and also followed in quantum complexity theory. As allowing mixed states changes the usual model of quantum computers, the parallel to quantum complexity theory is important, as our choice is legitimized there. Another dierence to the quantum coding theory to date is, that we focus on infinitely many coupled qubits. This has the same reasons as in classical coding theory and is further motivated in chapter3, section11 but not least, we were able to integrate. Last the common example of stabilizer codes as developed by Gottesman into our theory and to derive examples for our definitions from it. We could also include the examples of Ollivier and Tillich ([OT03] and [OT04]). Furthermore, the examples derived from stabilizer codes as developed by Gottesman have a very important property. This property allows us to interpret these stabilizer codes as mappings which hide a given state space in a larger one. The third main result is that we were able to show that the most important quantum algo-rithms, including stabilizer codes and the Shor algorithm, are in some sense commutative and thus classical. This can be done as quantum algorithms fit into the notion of quantum mea-surements, and our calculations imply that they can be represented as a coupling to a classical Bernoulli shift.
Survey of Chapters
The first two chapters are introductory. Inchapter1we give an overview of classical coding theory. We introduce the notions of alphabets, code spaces over such alphabets and coders, which are mappings between code spaces. Linear coders as well as convolutional coders will play an important role in the later chapters. The operator algebraic frame of this work and some notation are set inchapter2. The purpose ofchapter3a well known feature in quantum coding theory, namelyis to present stabilizer embeddings. We give an easier, base independent definition in the sections1to5
and give an example in section6. We also construct special stabilizer embeddings asm-blocks and stabilizer embeddings as introduced by Ollivier and Tillich and give examples for these embeddings in the sections7to10 we discuss the preliminaries of quantum code. Finally, spaces in section11. We start the next chapter,chapter4, by examining the definitions in literature. Then we define quantum alphabets and elements of quantum code spaces via algebraization of classical alpha-bets and codes in the sections4and5. This algebraization is inspired by a certain scheme that we explain in section2 also define quantum coders in section. We6and give first examples, namely q-higher power coders and q-1-block coders. Inchapter5coder out of stabilizer embeddings in sectionwe construct a q-1-block 1and give a nontrivial example of quantum convolutional coders constructed from the special stabilizer embeddings developed by Ollivier and Tillich in section2. We finish the chapter with a dis-cussion in section3, in which we reflect to what extent and how in a certain way these coders might be classical. The last chapter,chapter6a discussion of the preceding results as well as considera-, contains tions to what extent quantum coders and quantum algorithms make use of quantum mechanics. It starts with the definition of a measurement operator as well as the notion of essential com-mutativity in section1 second section describes quantum gates, special transformations. The that are used to implement quantum algorithms. In section3we prove that important classes of quantum algorithms are essentially commutative, i.e. that their measurement operator can be obtained from a coupling to a classical Bernoulli shift. The last section describes the mea-surement operator of the well known Grover quantum search algorithm.